Relax shader derivative test verification.

Relax shader derivative test verification.

- Disable GL_DITHER. Dithering only adds noise to result images.
- Add to dfdx and dfdy cases a flush-to-zero aware verification phase after
  preliminary verification to check if seemingly invalid derivatives could
  be caused by flushing or other relaxations allowed by the spec. This phase
  is NOT added to the fwidth cases since they operate with significantly higher
  thresholds which are likely to hide flushing anyway.

Bug: 18539021
Change-Id: Ic17909d30de25c8bae9d9996ce8df15c9ad5bc5e